Myocardial dysfunction in sepsis has been a central research focus of the Critical Care Medicine Department over the past 8 years. Clinical studies of myocardial function are limited by the confounding effects of changes in preload and afterload on ventricular function. The use of single myocytes in tissue culture has the major advantage of allowing assessment of contractility independent of changes in loading conditions. A system has been developed which utilizes spontaneously beating newborn rat myocytes, which are grown in Petri dishes, imaged under a phase- contrast microscope, linked to a television monitoring system, and data fed to an analytic system capable of quantitating the amplitude and velocity of myocyte contraction. Using this system it has been demonstrated that sera from septic patients depresses myocyte contraction in close quantitative and temporal association with the depression of left ventricular ejection fraction seen in vivo. Accordingly, we have sought to isolate and characterize this myocardial depressant substance (or substances) from human and from septic canine specimens; it appears to be a novel molecule with a molecular weight of greater than 10K daltons. The known mediators of sepsis (i.e., endotoxin, interleukins 1 and 2, and tumor necrosis factor) have been screened using the in vitro assay. The incidence of myocardial depressant activity has been assayed in a large clinical series of septic shock patients and found to be approximately 40%. It is hoped that specific identification of these myocardial depressant substances will lead directly to the creation of new therapeutic modalities for sepsis.